1. Field
This invention relates to signal decoding and, more particularly, to a system and method for video signal decoding using digital signal processing (DSP).
2. Related Art
Composite and S-video signal formats dominate home theater interconnect applications. Composite video signals, as the name implies, combine the three basic elements of a video picture (color, brightness, and synchronization signals) into a single composite signal. S-video is often incorrectly referred to as S-VHS since it first came into home use with the introduction of VHS tape format. Professionals prefer the name Y/C video rather than S-video since it is more clearly descriptive of the signal format. S-video signals comprise luminance (Y or luma) signals indicative of the black and white portion of the image (i.e., brightness), chrominance (C or chroma) indicative of the color portion of the image (i.e., hue and saturation), and vertical and horizontal synchronization signals. Manipulation of the composite and S-video signals allows image control. For instance, manipulation of Y and C components may sufficiently produce any image.
A decoder samples the composite and S-video signals using a clock locked to a synchronization signal (line locked decoding) or a color burst frequency. If locked to the color burst frequency, the decoder uses a phase-locked loop (PLL), a voltage controlled crystal oscillator (VCO), or a digitally tuned oscillator (DTO) to sample the composite and S-video signals. The sampling frequency must be controlled in real-time and in a manner that produces a stable clock otherwise the signals will produce an image with banding and other artifacts that adversely affect display quality.
If the input is a composite signal, the sampled signal is then processed through a Y/C separator to produce an S-video signal. A demodulator then demodulates the chroma portion of the S-video signal (original or processed). Typical demodulation techniques, like quadrature amplitude demodulation, multiply the chroma signal by a signal either in phase or 90 degrees out of phase with the color burst, and then filter the result using a low-pass filter. The result is a demodulated UV signal that is ready for color space conversion. Due to the sampling frequency control requirement, however, the above approach is not easily adaptable with common, off-the-shelf, digital signal processors, increasing the decoder's overall cost.
Accordingly, a need remains for an improved SYSTEM and method for video signal decoding using digital signal processing.